All matter is made of atoms.

Transcription

1 Chapter 5, Section 1 Key Concept: Atoms are the smallest form of elements. BEFORE, you learned All matter is made of atoms Elements are the simplest Substances NOW, you will learn * Where atoms are found and how they are named * About the structure of atoms * How ions are formed from atoms All matter is made of atoms. Think of all the substances you see and touch every day. Are all of these substances the same? Obviously, the substances that make up this book you re reading are quite different from the substances in the air around you. So how many different substances can there be? This is a question people have been asking for thousands of years. About 2400 years ago, Greek philosophers proposed that everything on Earth was made of only four basic substances air, water, fire, and earth. Everything else contained a mixture of these four substances. As time went on, chemists came to realize that there had to be more than four basic substances. Today chemists know that about 100 basic substances, or elements, account for everything we see and touch. Sometimes these elements appear by themselves. Most often, however, these elements appear in combination with other elements to make new substances. In this section, you ll learn about the atoms of the elements that make up the world and how these atoms differ from one another. reading tip Types of Atoms in Earth s Crust and Living Things Atoms of the element hydrogen account for about 90 percent of the total mass of the universe. Hydrogen atoms make up only about 1 percent of Earth s crust, however, and most of those hydrogen atoms are combined with oxygen atoms in the form of water. The graph below shows the types of atoms in approximately the top 100 kilometers of Earth s crust.

2 The distribution of the atoms of the elements in living things is very different from what it is in Earth s crust. Living things contain at least 25 types of atoms. Although the amounts of these atoms vary somewhat, all living things animals, plants, and bacteria are composed primarily of atoms of oxygen, carbon, hydrogen, and nitrogen. As you can see in the lower graph above, oxygen atoms account for more than half your body s mass. Check Your Reading What is the most common element in the universe? Names and Symbols of Elements Elements get their names in many different ways. Magnesium, for example, was named for the region in Greece known as Magnesia. Lithium comes from the Greek word lithos, which means stone. Neptunium was named after the planet Neptune. The elements einsteinium and fermium were named after scientists Albert Einstein and Enrico Fermi. Each element has its own unique symbol. For some elements, the symbol is simply the first letter of its name. hydrogen (H) sulfur (S) carbon (C)

3 The symbols for other elements use the first letter plus one other letter of the element s name. Notice that the first letter is capitalized but the second letter is not. aluminum (Al) platinum (Pt) cadmium (Cd) zinc (Zn) The origins of some symbols, however, are less obvious. The symbol for gold (Au), for example, doesn t seem to have anything to do with the element s name. The symbol refers instead to gold s name in Latin, aurum. Lead (Pb), iron (Fe), and copper (Cu) are a few other elements whose symbols come from Latin names. Atom Concentrations by Mass The Atomic Model Each element is made of a different atom. In the early 1800s British scientist John Dalton proposed that each element is made of tiny particles called atoms. Dalton stated that all of the atoms of a particular element are identical but are different from atoms of all other elements. Every atom of silver, for example, is similar to every other atom of silver but different from an atom of iron. Dalton s theory also assumed that atoms could not be divided into anything simpler. Scientists later discovered that this was not exactly true. They found that atoms are made of even smaller particles. The Structure of an Atom A key discovery leading to the current model of the atom was that atoms contain charged particles. The charge on a particle can be either positive or negative. Particles with the same type of charge repel each other they are pushed apart. Particles with different charges attract each other they are drawn toward each other. Atoms are composed of three types of particles electrons, protons, and neutrons. A proton is a positively charged particle, and a neutron is an uncharged particle. The neutron has approximately the same mass as a proton. The protons and neutrons of an atom are

4 grouped together in the atom s center. This combination of protons and neutrons is called the nucleus of the atom. Because it contains protons, the nucleus has a positive charge. Electrons are negatively charged particles that move around outside the nucleus Atoms are extremely small, about meters in diameter. This means that you could fit millions of atoms in the period at the end of this sentence. The diagram above, picturing the basic structure of the atom, is not drawn to scale. In an atom the electron cloud is about 10,000 times the diameter of the nucleus. Electrons are much smaller than protons or neutrons about 2000 times smaller. Electrons also move about the nucleus very quickly. Scientists have found that it is not possible to determine their exact positions with any certainty. This is why we picture the electrons as being in a cloud around the nucleus. The negative electrons remain associated with the nucleus because they are attracted to the positively charged protons. Also, because electrical charges that are alike (such as two negative charges) repel each other, electrons remain spread out in the electron cloud. Neutral atoms have no overall electrical charge because they have an equal number of protons and electrons.

5 Atomic Numbers If all atoms are composed of the same particles, how can there be more than 100 different elements? The identity of an atom is determined by the number of protons in its nucleus, called the atomic number. Every hydrogen atom atomic number 1 has exactly one proton in its nucleus. Every gold atom has 79 protons, which means the atomic number of gold is 79. Atomic Mass Numbers The total number of protons and neutrons in an atom s nucleus is called its atomic mass number. While the atoms of a certain element always have the same number of protons, they may not always have the same number of neutrons, so not all atoms of an element have the same atomic mass number. All chlorine atoms, for instance, have 17 protons. However, some chlorine atoms have 18 neutrons, while other chlorine atoms have 20 neutrons. Atoms of chlorine with 18 and 20 neutrons are called chlorine isotopes. Isotopes are atoms of the same element that have a different number of neutrons. Some elements have many isotopes, while other elements have just a few. check your reading How is atomic mass number different from atomic number?

6 A particular isotope is designated by the name of the element and the total number of its protons and neutrons. You can find the number of neutrons in a particular isotope by subtracting the atomic number from the atomic mass number. See the diagram above. For example, chlorine-35 indicates the isotope of chlorine that has 18 neutrons. Chlorine-37 has 20 neutrons. Every atom of a given element always has the same atomic number because it has the same number of protons. However, the atomic mass number varies depending on the number of neutrons. Atoms form ions. An atom has an equal number of electrons and protons. Since each electron has one negative charge and each proton has one positive charge, atoms have no overall electrical charge. An is formed when an atom loses or gains one or more electrons. Because the number of electrons in an ion is different from the number of protons, an ion does have an overall electric charge. Formation of Positive Ions Consider how a positive ion can be formed from an atom. The left side of the illustration below represents a sodium (Na) atom. Its nucleus contains 11 protons and some neutrons. Because the electron cloud surrounding the nucleus consists of 11 electrons, there is no overall charge on the atom. If the atom loses one electron, however, the charges are no longer balanced. There is now one more proton than

7 there are electrons. The ion formed, therefore, has a positive charge. Notice the size of the positive ion. Because there are fewer electrons, there is less of a repulsion among the remaining electrons. Therefore, the positive ion is smaller than the neutral atom. Positive ions are represented by the symbol for the element with a raised plus sign to indicate the positive charge. In the above example, the sodium ion is represented as Na+. Some atoms form positive ions by losing more than one electron. In those cases, the symbol for the ion also indicates the number of positive charges on the ion. For example, calcium loses two electrons to form an ion Ca 2+, and aluminum loses three electrons to form Al 3+. Check Your Reading What must happen to form a positive ion? Formation of Negative Ions The illustration below shows how a negative ion is formed. In this case the atom is chlorine (Cl). The nucleus of a chlorine atom contains 17 protons and some neutrons. The electron cloud has 17 electrons, so the atom has no overall charge. When an electron is added to the chlorine atom, a negatively charged ion is formed. Notice that a negative ion is larger than the neutral atom that formed it. The extra electron increases the repulsion within the cloud, causing it to expand.

8 Negative ions are represented by placing a minus sign to the right and slightly above the element s symbol. The negative chloride ion in the example, therefore, would be written as Cl. If an ion has gained more than one electron, the number of added electrons is indicated by a number in front of the minus sign. Oxygen (O), for example, gains two electrons when it forms an ion. Its symbol is O 2. Questions for Chapter 5, Section 1 KEY CONCEPTS 1. Which two atoms are most common in Earth s crust? in the human body? 2. What are the particles that make up an atom? 3. What happens when an atom forms an ion? CRITICAL THINKING 4. Infer Magnesium and sodium atoms are about the same size. How does the size of a magnesium ion with a 2+ charge

9 compare with that of a sodium ion with a single + charge? 5. Compare The atomic number of potassium is 19. How does potassium-39 differ from potassium-41? CHALLENGE 6. Analyze When determining the mass of an atom, the electrons are not considered. Why can scientists disregard the electrons?